Calculation of lathing for corrugated sheeting: diagrams and standards

The quality and durability of a roof made of corrugated sheets directly depend on how competently the calculation of the sheathing was carried out. Many inexperienced developers mistakenly believe that it is enough to simply nail the boards to the rafters, ignoring the wind and snow loads characteristic of a particular region. Such negligence often leads to deformation of the metal covering, leaks and even destruction of the entire roof structure in the first winter season. Lathing performs the function of a supporting frame that evenly distributes the weight of snow and gusts of wind over the entire area of the slope.

In this material we will analyze in detail how to calculate the lathing pitch for corrugated sheeting, what materials are best to use and what regulatory documents to rely on when designing. You'll learn the differences between solid and sparse installation patterns, and understand why the roof angle plays a decisive role in choosing the spacing of the beams. Correctly selected sheathing pitch is a guarantee that your roof will last for decades without the need for complex repairs.

Before you start purchasing lumber, it is necessary to take accurate measurements and determine the type of profiled sheet used. Different brands of corrugated sheets have different load-bearing capacities, which dictates their own requirements for the frequency of support elements. Load capacity roofing is a complex indicator depending on the thickness of the metal, wave height and quality of fasteners. Ignoring these parameters can lead to sagging of the metal between the boards, which will break the tightness of the joints.

Selection of materials for the base device

The basis for fastening the profiled sheet is most often an edged board or timber that has been kiln dried. The use of raw wood is strictly not recommended, since during the drying process the material will begin to deform, twist and may tear the waterproofing film or screws. The optimal moisture content for lumber is considered to be 20%. To create a reliable frame, a board with a section of 32x100 mm or 50x50 mm. In some cases, especially when installing continuous sheathing, it is permissible to use slabs OSB-3 or moisture-resistant plywood, however, this significantly increases the cost of the structure.

When choosing between timber and boards, it is important to consider material savings and the weight of the structure. A board with a thickness of 25-32 mm is the “gold standard” for private house construction, as it is lighter and cheaper than timber, but at the same time provides sufficient rigidity. If you plan to use thinner material, for example, inch (25 mm), then the rafter spacing should be reduced, or the lathing spacing should be more frequent. Beam section 50x50 mm is more often used in regions with high snow loads or when using heavy types of roofing.

⚠️ Attention: Never use a slab or unedged board with bark for lathing. Bark is an ideal breeding ground for bark beetles and mold, which will quickly cause the entire roofing system to rot from the inside out.

An important stage of preparation is the treatment of wood with fire-retardant compounds. Even if you buy material that has already undergone factory processing, craftsmen recommend applying an additional layer of antiseptic to the cut points and ends. This will create a reliable barrier to moisture and microorganisms. Fire protection not only extends the life of wood, but also increases the fire safety of the entire building, which is especially important for wooden houses.

The influence of the roof angle on the pitch of the sheathing

The angle of inclination of the slope is the main parameter that determines the installation scheme of the supporting frame. The smaller the angle of inclination, the greater the area of ​​contact of snow with the roof and the higher the risk of its accumulation. Consequently, low-slope roofs require more frequent sheathing spacing or even a continuous deck. For roofs with a slope of less than 15 degrees, it is often recommended to continuous sheathingto prevent the profiled sheet from bending under the weight of wet snow.

As the angle of inclination increases, the snow load decreases, as the snow rolls off the surface more easily. In such cases (angle more than 30-45 degrees), it is allowed to make the sheathing spacing sparser, which allows saving lumber. However, another factor comes into force here - wind load. On steep slopes, the roof windage is higher, so the frame must be rigid enough to withstand the tearing forces of the wind. The calculation must be made taking into account the balance between snow and wind loads for a specific area.

There is a direct relationship: the higher the roof, the higher the requirements for tightness and frequency of supports. If the slope angle is less than 12 degrees, experts strongly advise waterproofing using special under-roof membranes and lathing as often as possible. This will prevent snow from pushing inside the room during heavy snowstorms. Sealing joints in this case also require special attention and the use of sealing tapes.

Installation schemes: sparse and continuous lathing

In construction, two main schemes for arranging the base for corrugated sheeting are used: sparse (step-by-step) and continuous. The choice between them depends on the brand of corrugated sheet, the angle of inclination and climatic conditions. Sparse lathing is a series of parallel boards laid at a certain pitch. This is the most common option, which provides good ventilation of the under-roof space and saves materials.

Continuous sheathing is a flooring made of boards laid close to each other (with a gap of 2-3 cm for thermal expansion), or made of sheet materials (plywood, OSB). This type of base is necessary for soft roofs, but is also used under corrugated sheeting on very gentle slopes or in places with an increased risk of leaks (valleys, eaves, around chimneys). Ventilation gap in continuous sheathing is critical for removing condensation that forms on the back side of the metal.

Features of installation in valleys

In areas of the internal corners of the roof (valleys), a continuous sheathing with a width of at least 600 mm from the axis of the corner in each direction is always performed. This is necessary to securely fasten additional waterproofing elements and prevent leaks in the most vulnerable areas of the roof.

A combined approach is often the most rational solution. For example, the main field of the slope can be covered with sparse lathing with a pitch of 300-400 mm, and the eaves overhangs, ridges and areas around the attic windows can be reinforced with continuous flooring. This allows you to optimize the budget without sacrificing the reliability of the design in critical components. Curtain board should always be thicker than the ordinary one, since it bears the load from the gutters and the first row of the roof.

Lathing pitch table depending on the brand of corrugated sheeting

Each brand of profiled sheet has its own rigidity characteristics, which are determined by the wave height and metal thickness. To calculate the pitch of the sheathing, it is necessary to refer to the technical characteristics of a particular manufacturer, however, there are average standard values. Below is a table to help you navigate in choosing the distance between frame elements for popular brands of corrugated sheeting.

Brand of corrugated sheet	Wave height, mm	Angle of inclination, degrees.	Recommended lathing pitch, mm
S-8, S-10	8-10	15° or more	Continuous sheathing
S-20, S-21	20-21	15° or more	Up to 300
NS-35	35	15° or more	Up to 500
N-60, N-75	60-75	8° or more	Up to 1000

The table shows that for sheets with low wave heights (C-8, C-10) an almost continuous flooring is required even at medium inclination angles, since their load-bearing capacity without frequent support is low. At the same time, load-bearing profiled sheets (grade H) with a high trapezoidal wave allow you to make lathing pitches of up to 1 meter or more, which significantly saves the budget on lumber.

When using corrugated sheeting with a thickness of less than 0.5 mm, the sheathing pitch should be reduced regardless of the brand, since thin metal is more susceptible to deformation. It is also worth considering that as the sheathing pitch increases, it is necessary to use more powerful fasteners and more often attach the sheets to each board, and not through a wave. Fasteners must correspond to the loads, otherwise the sheet may be torn off by the wind.

Installation technology and calculation of the amount of materials

Installation of the sheathing begins with the installation of the eaves board, which must be strictly parallel to the eaves of the building. The first board (cornice) is often made thicker than the others by the height of the corrugated sheet wave in order to level the plane of the slope. Next, moving from bottom to top, the remaining elements are attached with a calculated step. It is most convenient to control the distance between boards using a template (a block of the required length), which speeds up the process and eliminates errors.

To calculate the amount of materials, you need to know the area of the slopes and the pitch of the sheathing. The formula is simple: the length of the slope is divided by the pitch of the sheathing, obtaining the number of rows, then multiplied by the length of the slope and the number of slopes. Don't forget to add a percentage for scraps. It is also important to correctly calculate the amount nails or screws. For each fastening point there are usually two nails (or one long self-tapping screw), driven into the edges of the board to avoid cracking.

The fastening of the boards to the rafters must be reliable. If nails are used, their length should be equal to twice the thickness of the sheathing board. When using self-tapping screws, it is important not to overtighten them so as not to split the wood. In places where boards join (if the length of the slope is greater than the length of the lumber), the joint should fall on the rafter leg. Docking in the air unacceptable, since this is the weak point of the structure.

⚠️ Attention: When installing the sheathing, be sure to leave a ventilation gap between the waterproofing film and the board. To do this, the height of the counter-lattice (the bar on which the board is attached) must be at least 30-50 mm. Lack of ventilation will lead to rotting of the rafters.

Common mistakes and expert recommendations

One of the most common mistakes is saving on the section of the board. An attempt to use a board with a thickness of 20 mm instead of 32 mm on a large rafter pitch results in the board breaking when walking on the roof during installation or maintenance. This is not only dangerous, but also disrupts the geometry of the roof. Safety when working at height is the number one priority, so the frame must be able to withstand the weight of a person with a margin.

Another common mistake is ignoring the quality of waterproofing. Even the strongest sheathing will not save a house from leaks if the vapor-permeable membrane underneath is torn or the sealing around the pipes is not done correctly. Corrugated sheeting is a cold material, and condensation always forms on its inner surface. The task of the sheathing and counter-lattice is to ensure that this moisture is drained down into the drain, preventing it from getting on the insulation.

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The main secret of a durable roof is not so much in the brand of corrugated sheeting, but in a well-assembled “pie”: proper ventilation, reliable waterproofing and precisely calculated sheathing pitch.

It is also worth mentioning the mistake when choosing fasteners. Using ordinary black wood screws instead of galvanized roofing screws with EPDM washers will lead to rapid corrosion of the fasteners and the appearance of rusty streaks on the roof after 2-3 years. Roofing screws They have a special coating and a rubber gasket that seals the hole and prevents water from penetrating inside.

Is it possible to use unedged boards for sheathing?

Theoretically, it is possible if it is thoroughly cleaned of bark and treated with an antiseptic, and one side is leveled. However, in practice this is labor-intensive and the savings are dubious. Unedged boards often have different thicknesses, which will lead to an uneven roof plane and difficulties when installing corrugated sheets. It is better to use a standard kiln-drying edged board.

Is there a need for a gap between the sheathing boards?

Yes, when using edged boards, the gap is necessary to compensate for the thermal expansion of the wood and provide ventilation. Typically, boards are laid with a step calculated according to the formula, which automatically creates a gap. When using sheet materials (OSB, plywood), the technological gap between the sheets should be 3-5 mm.

What is the lathing pitch for S-21 corrugated sheeting?

For C-21 corrugated sheets with a roof slope angle of more than 15 degrees, the recommended sheathing pitch is 300-650 mm, depending on the snow load. For flat roofs (less than 15 degrees), the pitch should be reduced to 300 mm or the sheathing should be made continuous.

What to do if the pitch of the rafters does not match the pitch of the sheathing?

The pitch of the sheathing is tied to the pitch of the rafters. Sheathing boards are fastened across the rafters. If the rafters are rare (more than 80-100 cm), then for the sheathing you need to take a thicker board (50 mm) or timber so that it does not sag between the rafter legs. In critical cases, additional supports are installed or the pitch of the rafters is reduced.